The present invention relates to a light-sensitive mixture which contains a water-insoluble binder, which is soluble in aqueous-alkaline solutions, and a light-sensitive compound, as well as to a light-sensitive copying material composed of a support and a light-sensitive layer constituted by the mixture, and to a process for the preparation of a printing form by exposing, developing and heating the copying material.
Processes for the preparation of printing forms by heating or baking of the image layer of exposed and developed copying materials have been disclosed, for example, in British patent specifications Nos. 1,151,199 and 1,154,749. In these processes, a light-sensitive copying material, which is either positive-working and preferably contains an o-quinone-diazide or is negative-working and preferably contains a p-quinone-diazide, is exposed imagewise, developed and then heated to a temperature above 180.degree. C. for such a period and to such a temperature level that the image background, previously developed cleanly, is stained by thermal decomposition products of the image layer. Subsequently, the image background is cleaned again by once more treating it with developer solution. Normally, temperatures of 220.degree.-240.degree. C. are used for heating, and the duration of heating is about 5-60 minutes. This after-treatment achieves a hardening of the printing layer and hence a considerable extension of the print run. When the baking temperatures are at the upper limit of the indicated range, the required baking times are of course relatively short. It has been found that the use of relatively high temperatures in this process is disadvantageous. On the one hand, these temperatures cause the formation of a relatively firmly adhering precipitate in the background areas, and this can be cleanly removed only by means of relatively aggressive solutions, and by so doing there is also a risk of an attack on the printing stencil. Secondly, at the relatively high baking temperatures or with the relatively long baking times, there is a risk that the conventionally used supports of aluminum will be deformed and that the printing form obtained will be damaged or rendered useless.
As described in German Offenlegungsschrift No. 2,626,473, the first-mentioned disadvantage can be avoided by coating the background surface before baking with a layer of a water-soluble organic substance or an inorganic salt, which layer can be readily washed off after baking. To prevent a deformation of the support, it would be desirable however, to be able to bake at lower temperatures.
German Offenlegungsschrift No. 2,547,905 has disclosed a light-sensitive mixture of the generic type described here which, in addition to an o-quinone-diazide and an alkali-soluble binder, contains a low-molecular phenol derivative, for example, 2,3,4-trihydroxy-benzophenone. The phenol derivative has the purpose of improving the adhesion of the light-sensitive layer to the support, independently of a possible baking step.